The objective of this work is the detailed description of the hydroxylation reactions that are involved in the biosynthesis of the neurotransmitters, dopamine and norepinephrine. Recently, our emphasis has been on dopamine beta-hydroxylase and two enzymes involved in the synthesis of tetrahydrobiopterin, dihydropteridine reductase and dihydrofolate reductase. Dihydrofolate reductase activity has been found in brain and shown to reduce dihydrobiopterin. The ability of dihydropteridine reductase to function in folate metabolism has also been demonstrated. With regard to dopamine-beta-hydroxylase, the emphasis has been to study the mechanism for catechol oxidation by this enzyme. BIBLIOGRAPHIC REFERENCES: Hoeldtke, R. and Kaufman S.: Bovine adrenal tyrosine hyroxylase. Purification and properties. J. Biol. Chem. 252: 316-3169, 1977. Katz, I., Lloyd, T. and Kaufman, S.: Studies on phenylalanine and tyrosine hydroxylation by rat brain tyrosine hydroxylase. Biochim. Biophys. Acta 445: 567-578, 1976.